1. Field of the Invention
This invention relates broadly to methods for investigating subsurface earth formations. More particularly, this invention relates to borehole tools and methods for determining the permeability and other hydraulic properties of earth formations surrounding boreholes.
2. State of the Art
The determination of permeability and other hydraulic properties of formations surrounding boreholes is very useful in gauging the producibility of formations, and in obtaining an overall understanding of the structure of the formations. For the reservoir engineer, permeability is generally considered a fundamental reservoir parameter, the determination of which is at least equal in importance with the determination of porosity, fluid saturations, and formation pressure. When obtainable, cores of the formation provide important data concerning permeability. However, cores are difficult and expensive to obtain, and core analysis is time consuming and provides information about very small sample areas. In addition, cores, when brought to the surface may not adequately represent downhole conditions. Thus, in situ determinations of permeability which can quickly provide determinations of permeabilities over larger portions of the formation are highly desirable.
The primary technique presently used for in situ determination of permeability is the "drawdown" method where a probe of a formation testing tool is placed against the borehole wall, and the pressure inside the tool (e.g., at a chamber) is brought below the pressure of the formation, thereby inducing fluids to flow into the formation testing tool. By measuring pressures and/or fluid flow rates at and/or away from the probe, and processing those measurements, determinations regarding permeability are obtained. These determinations, however, have typically been subject to large errors. Among the reasons for error include the fact that liberation of gas during drawdown provides anamolous pressure and fluid flow rate readings, and the fact that the properties of the fluid being drawn into the borehole tool are not known accurately. Another source of error is the damage to the formation (i.e., pores can be clogged by migrating fines) which occurs when the fluid flow rate towards the probe is caused to be too large.
Another technique which has been disclosed (although not used) for making in situ permeability measurements is the injection of fluids into the formation. An early mention of the use of injection is found in U.S. Pat. No. 2,747,401 to Doll. Doll discusses a multi-probe tool, and proposes the injection of either oil or water into the formation while monitoring the pressures at the observation probe in order to determine permeability. Doll also discusses withdrawal of fluid from the formation. More recent patent disclosures of permeability testing tools include U.S. Pat. No. 4,742,459 to Lasseter, and U.S. Pat. No. 4,860,581 to Zimmerman et al.; both of which are assigned to the assignee hereof. While both patents recognize that fluid injection is a possibility, both patents primarily disclose apparatus which withdraw fluid from the formation. The Zimmerman et al. patent mentions that in the drawdown method, it is essential to limit the pressure reduction so as to prevent gas liberation. In order to prevent gas liberation, Zimmerman et al. propose a flow controller which regulates the rate of fluid flow into the tool.
While the tools of the art are somewhat successful in obtaining valuable permeability information, they are still subject to errors. Moreover, the prior art tools do not address the chacterization of multiphase flow in the formation (e.g., oil and water), and therefore are incapable of providing determinations of effective endpoint permeabilities and relative permeabilities.